This invention relates to an improved method of dehalogenating particulate ceramic powders, and in particular relates to an improved method of defluorinating compositions comprising uranium compounds in a manner substantially insuring absence of recombined fluoride ions with the compositions. Still further this invention relates to an improvement in the form of a baffle means forming a constricted gas flow zone in a rotary calciner having a heating zone and a cooling zone. The constricted zone has a high flow velocity of a defluorinating atmosphere therethrough serving to prevent back diffusion to the cooling zone of the calciner of gaseous impurities removed from the treated compositions in the heating zone of the calciner. This prevents recombination of gaseous impurities with the treated compositions in the cooling zone.
Uranium oxides have various utilities in the nuclear industry. One of the very important utilities of uranium oxides, especially uranium dioxide, is in nuclear power plants as a fuel in the generation of electrical power.
Since it is necessary to enrich uranium with the U-235 isotope for proper operation of the nuclear fuel in the nuclear power plant, this enrichment step is conveniently accomplished with the uranium in the form of uranium hexafluoride. But this compound must then be converted to a solid form such as an oxide for convenience of introduction to the nuclear reactor.
Uranium dioxide is typically produced from a hydrolysis-precipitation-reduction reaction which starts with uranium hexafluoride, and the uranium dioxide can be employed alone or in a mixture with other ceramic additives such as gadolinium oxide, plutonium oxide, silicon dioxide and titanium dioxide as a fuel for nuclear reactors.
In greater detail, one representative method of preparing uranium dioxide from uranium hexafluoride has uranium hexafluoride reacted with water to hydrolyze the fluoride and form a water solution of uranium oxyfloride and an acid. This water solution is reacted wtih ammonia to yield a precipitate of ammonium diuranate in a slurry. The ammonium diuranate slurry is converted to a dry particulate form of uranium dioxide by heating in wet hydrogen which achieves partial defluorination and reduction of the ammonium diuranate to uranium dioxide.
Other methods of converting uranium hexafluoride to uranium dioxide produce a uranium dioxide having varying amounts of fluoride impurities, typically impurity contents of about 3 percent to about 5 percent fluoride. Since the uranium dioxide is in a particulate form it is conveniently charged to a rotary calciner which has in a sequence a feeding (or preheating) zone, an elevated temperature or heating zone and a cooling zone from the uranium dioxide inlet end to the uranium dioxide outlet end of the calciner. The calciner uses gravity for working the particulate uranium dioxide from the inlet end to the outlet end. Typically a gaseous atmosphere is passed in the opposite direction to the movement of the uranium dioxide in the calciner. The gaseous atmosphere typically achieves defluorination of the uranium dioxide while controlling the final oxygen-to-metal ratio of the uranium dioxide. The gaseous atmosphere can have various compositions, such as wet hydrogen, dry hydrogen, a mixture comprising carbon dioxide and carbon monoxide, etc. One improved atmosphere used for this treatment of uranium dioxide is a mixture comprising hydrogen and carbon dioxide as disclosed in U.S. patent application Ser. No. 62,308 entitled "Ceramic Defluorination and Reduction Process" , now abandoned in favor of continuation-in-part application Ser. No. 258,738, filed May 9, 1973. The foregoing application was filed August 10, 1970 in the name of Y. Nivas and assigned to the same assignee as the present application.
In the operation of a rotary calciner, it is preferred to pass the gaseous atmosphere countercurrent to the direction of flow of the particulate composition being treated in the calciner. In this manner the incoming atmosphere is initially in contact with the particulate composition having the lowest fluoride content and as the gas composition passes toward the gas outlet of the rotary calciner, it gains in impurity content removed from the particulate composition. In this manner the incoming atmosphere is in contact with the lowest impurity composition near the treated composition outlet of the calciner and there results a minimum impurity content in the particulate composition as it is removed from the treated composition outlet of the calciner.
However in spite of the desirable performance of this process, it has been determined that impurities removed from the particulate composition and carried in the gaseous atmosphere can back diffuse in the calciner (i.e., go against the direction of flow of the gaseous atmosphere through the calciner) so that the impurities come in contact with the treated particulate composition in the cooling zone of the calciner. This produces a problem since the impurities recombine with the particulate composition which lowers the efficiency of the defluorination process.